I have gum recession and my dentist suggested bonding the areas that are most severe. How long does does bonding last?

First you need to find out why there is a recession, to get to root cause of problem, it can be bone loss, bite and occlusion, and many other factors..... Once the recession is diagnosed, it can be treated via gum grafting, to eliminate sensitivity, or better yet you can use pinhole technique ( newer gum grafting technique), in which the gum is pulled down vs grafting in which the dentist cuts gum from palate or other recipient site to graft it in the area of the recession.
If you do not want surgery then dental bond can be an alternative, it is done in one visit, conservatively, and you will be done. The technique for dental bond to the root is technique sensitive, so the root does not look over-contoured and the root is sealed completely with no leakage of bacteria!
According to ADA ( american dental association) the dental bond can last 3 to 10 years! However the good dentist need to check your occlusion also, to make sure your bite particularly in the area of recession is balanced.
Dental bond is a great technique to conservatively restore the root area, however the expert dentist must look at the cosmetic aspect of the treatment to make sure the tooth will not look much longer.
Now days there is even pink dental composite to bond to your root area which mimics gums, the color might not be exact but it does help in areas that would show elongated length of the tooth in the Aesthetic zone ( where teeth show in your SMILE).
Best of Luck

Hi:It is difficult to predict.  Normally bonding can last anywhere between 3-7 years.  However, if you are a clenched or a grinder you can break off the bonding very easily.Best,Dr. Maddahi

A better question is, "Why are my gums receding?". Are you  using too stiff of a tooth brush bristle? Are you pressing too hard? Typically gums recede if the underlying bone is thin. Bonding on a root surface can be problematic because the bonding material does not adhere very well. It will stay in place from one to ten years.